This invention relates to a method of image emphasizing especially suitable for image processing systems for images such as radiographic images.
A known method of sharpening radiographic images as exemplified in, for example, Japanese patent unexamined publication No. 56-11035 employs an unsharp or unsharped masking processing in which the emphasizing coefficient is determined or changed in accordance with a density value of an original i.e., unprocessed or untreated image (in terms of a pixel image) or a local mean density value of the original image obtained by averaging neighbouring pixels of the original image (also in terms of pixel images).
The known method, however, takes no account of a capability of interactive processing which can interactively change the emphasizing coefficient to permit real-time changes in picture quality.
Consequently, values of parameters for determining picture quality must be set in advance by a doctor or an examiner and besides the processing time extends to amounts up to approximately 90 to 180 seconds, resulting in difficulties in obtaining optimum picture quality for each image. More particularly, an unsharp masking processing employed by the known method is formulated as follows: EQU X'.sub.ij =.beta.(X.sub.ij)(X.sub.ij -X.sub.ij)+X.sub.ij EQU (X.sub.ij =.sub.k .SIGMA..sub.l .SIGMA.X.sub.i.+-.k,j.+-.l /N) (1)
where an original image (implicitly representative of its density) is represented by Xij, (i.j, represent a column and row of an image in a two-dimensional matrix, respectively) an unsharp image or unsharped image (also implicitly representative of its density) by Xij, an emphasizing coefficient by .beta.(Xij), and a processed image by X'ij.
In order to determine picture quality according to this processing,
(1) a window size (values of k and l which represent a column and row of the window, respectively) necessary for averaging must be set in advance, and
(2) a maximum value and a function form of the emphasizing coefficient .beta.(Xij) must be set in advance. But these parameters may possibly be changed in accordance with the purposes of image viewing and the preference of an image viewer and the capability of interactively setting the parameters is therefore indispensable. In addition, when it is desired to define the emphasizing coefficient in a more sophisticated manner by using two variables, for example, the pre-setting of the parameters becomes more difficult.
Incidentally, when the density value is specified to correspond to an imaging region (such as a bone or a muscle to be radiographically imaged), the aforementioned known method is considered to be effective for such an imaging portion because of its capability to change the degree of emphasizing in accordance with the imaging portion. But, for an instance where there is no fixed correspondence between density value and imaging portion or where the degree of emphasizing is desired to be changed for the same density value, this known method is unsuitable, facing problems that insufficient emphasizing occurs for an imaging portion which is flat and subject to a gradual change in density value of its image and that excessive emphasizing occurs for an imaging portion which corresponds to an edge portion subject to a rapid change in density value of its image. Further, since a simple averaging is the basis for formation of an unsharp image, the known method suffers from generation of an artifact at the edge portion in addition to the excessive emphasis.